For many years, the electric horns commonly used on automotive vehicles have been of the type which generate sound by vibration of a diaphragm driven by an electromagnet motor. The horn typically comprises a housing with the diaphragm peripherally clamped thereto forming a motor chamber. The coil of the electromagnet is mounted within the chamber and a magnetic pole piece on the housing extends axially of the coil. A magnetic plunger on the diaphragm extends toward the pole piece for imparting motion to the diaphragm in response to periodic energization of the coil. The diaphragm provides a resilient suspension of the plunger for reciprocating motion relative to the coil; it has a spring characteristic whereby the diaphragm and the mass carried by it have a resonant frequency of mechanical vibration. The coil is energized from the vehicle battery through a mechanically actuated switch which is alternately opened and closed by movement of the plunger with the diaphragm. A vehicle horn of this kind is described in the Wilson et al U.S. Pat. No. 4,813,123 granted Mar. 21, 1989.
Although vehicle horns of the type just described have been eminently successful in the automotive industry for many years, there have been certain problems which, for a long time, have seemingly defied solution. One such problem is that of manufacturing the horn with sufficiently exacting mechanical and electrical relationships so as to obtain a high degree of operating efficiency. Particularly, such horns have not been readily adjustable to obtain operation at the maximum achievable sound pressure level for a given input power.
A vehicle horn which employs a solid state driver circuit for the horn coil is disclosed and claimed in U.S. Pat. No. 5,049,853, issued Sep. 17, 1991 assigned to the assignee of this application. In that horn, the driver circuit is adapted to energize the horn coil to cause vibrations of the diaphragm at its resonant frequency. The solid state driver has an electronic timer adjustable to the frequency of the diaphragm assembly and switches a solid state power output stage to drive the diaphragm synchronously with the timer frequency.
Another vehicle horn which employs a solid state driver circuit is disclosed and claimed in copending application Ser. No. 684,693 filed Apr. 12, 1991 by Carl R. Wilson et al and assigned to the assignee of this application. The horn described in that patent application has an energizing circuit and electromagnet for driving a diaphragm assembly at its resonant frequency of mechanical vibration. The energizing circuit generates a DC pulse train for energizing the coil of the electromagnet to drive the diaphragm. The circuit has an adjustment for setting the pulse repetition rate of the pulse train substantially equal to the resonant frequency. This setting is made as a factory adjustment. The rated supply voltage is applied to the horn and the horn is operated over a frequency range sufficiently broad to include that frequency which produces a maximum sound pressure level output from the horn. The sound pressure level output is monitored to determine the frequency which produces the maximum output. The horn energizing circuit is adjusted to produce a pulse train at that frequency.
The vehicle horn of the type referred to above, is typically fitted with either a resonant projector or a resonator to propagate sound pressure waves into the atmosphere. The resonant projector is a trumpet-like device comprising a spiral passageway to define an air column of increasing cross-section from the inlet end at the diaphragm to the outlet end at a bell. A horn with this acoustic coupling device is commonly known as a "seashell" horn. It generates sound by the free vibration of the diaphragm. The resonator is a vibratory plate of circular configuration which is mounted at its center on the diaphragm and plunger. In this device, the horn is energized so that the plunger strikes the pole piece during each cycle of diaphragm motion; the force of the strike is transferred to the center of the circular resonator causing it to vibrate at its natural frequency and generate sound pressure waves which are propagated directly into the surrounding atmosphere without any intermediate coupling device. This type of horn is commonly known as a "vibrator" horn. The two horns produce distinctly different sounds. A vehicle is sometimes provided with a pair of seashell horns or a pair of vibrator horns. To produce the desired sound, one horn of each pair is designed for relatively low frequency and the other for high. For the vibrator horns this is typically 350 Hz and 440 Hz. For seashell horns it is 400 Hz and 500 Hz.
In such vehicle horns, it is desired to operate the horn so that the diaphragm is vibrated at its natural resonant frequency. This provides the maximum sound pressure level output from the horn for a given input power.
A general object of this invention is to provide a vehicle horn with a solid state energizing circuit which permits adjustment for operation with high efficiency at maximum sound pressure level and to overcome certain disadvantages of the prior art.